Fluorescent lamps and high-intensity discharge lamps (HID) are popular and commonly used in many lighting systems. These lamps produce light when they are energized by a suitable power source, as a consequence of the well known gas discharge phenomenon. They require a high power level to initiate the light producing gas discharge effect but thereafter may be operated at substantially reduced power levels. This characteristic of fluorescent lamps and high-intensity discharge lamps allows various designs of energy saving lighting control systems which are capable of responding to the power demand of a load of these lamps by switching from providing a full voltage to providing a reduced voltage, or vice versa.
For example, U.S. Pat. No. 4,513,224 issued to Thomas sets forth a FLUORESCENT-LIGHTING-SYSTEM VOLTAGE CONTROLLER having a three phase transformer which includes three auto-transformer windings, each used for developing two reduced voltages. Three contactors selectively couple the full voltage and reduced voltages to the lighting systems. The contactors are switched in closed transition fashion to avoid power interruptions. An additional contactor is used for opening the winding neutral connections during the switching operation.
U.S. Pat. No. 4,766,352 issued to Widmayer sets forth a METHOD AND APPARATUS FOR STARTING AND OPERATING FLUORESCENT LAMP AND AUXILIARY BALLAST SYSTEMS AT REDUCED POWER LEVELS in which a capacitor is selected to provide effective starting of rapid start, preheat, and instant start type fluorescent lamps. A standard AC operated ballast transformer is operated at reduced power levels to achieve energy conservation. The capacitor is connected in series with the ballast primary winding and is selected to have a value producing ferro-resonance within the ballast transformer primary circuit.
U.S. Pat. No. 4,527,099 issued to Capewell, et al. sets forth a CONTROL CIRCUIT FOR GAS DISCHARGE LAMPS which includes anti-parallel connected controlled rectifiers connected in series with an AC source and the ballast. A current limiting and energy diversion capacitor is connected in series with the rectifiers and in shunt with the ballast. The controlled rectifiers of the series and shunt switching assemblies are controlled such that in any given half wave, the related controlled rectifier of the shunt switching means turns on to discharge a capacitor into the normally conducting controlled rectifier of the series switching means to produce a notch in the voltage waveform applied to the inductive ballast.
U.S. Pat. No. 4,464,606 issued to Kane sets forth a PULSE WIDTH MODULATED DIMMING ARRANGEMENT FOR FLUORESCENT LAMPS which includes a base driven high frequency push-pull transistorized inverter circuit used for energizing the lamps. The inverter is pulse width modulated to effect dimming. Transitory circuitry is provided for insuring rapid turn on and off of the inverter transistors. A photoresponsive sensor responds to ambient light and illumination produced by the lamps to control the pulse width modulator accordingly.
U.S. Pat. No. 4,435,670 issued to Evans, et al. sets forth an ENERGY CONSERVING INSTANT START SERIES SEQUENCE FLUORESCENT LAMP SYSTEM WITH OVERCURRENT PROTECTION which includes a power reducing capacitor connected in series with one or both of the lamps in a two-lamp system. A protective device is connected within the circuit of the first lamp such that the high current flow produced by failure of the second lamp to start activates the protective device and prevents the system from being damaged.
U.S. Pat. No. 4,434,388 issued to Carver, et al. sets forth an ELECTRICAL LIGHTING CONTROLLER which is connected between a power line and a bank of lamps or other electrical energy consuming devices. The output level applied to the lamps is controlled by a variable autotransformer having a drive motor which in turn is controlled by an amplifier comparator circuit.
U.S. Pat. No. 4,339,690 issued to Regan, et al. sets forth an ENERGY SAVING FLUORESCENT LIGHTING SYSTEM which includes a reactants-modifying capacitor coupled in series with first and second fluorescent lamps. A filament switch is operative to conduct filament heating current during the starting of the first lamp. The filament switch is coupled between filaments at opposite ends of the first fluorescent lamp and triggers to a low impedance state in response to the lamp starting voltage.
U.S. Pat. No. 4,256,993 issued to Morton sets forth an ENERGY SAVING DEVICE FOR RAPID-START FLUORESCENT LAMP SYSTEM which is connected in a series with one lamp of a two-lamp rapid start fluorescent light system. The device includes a normally closed relay within the electrode circuit of one of the lamps and a power reducing capacitor in shunt with one of the relay's contacts. Upon turning on the system, a solid state time delay and relay coil energizing circuit is actuated which opens the relay contacts only after the lamps have been started, placing the shunt capacitor in series with the operating lamps to reduce the nominal power consumption.
U.S. Pat. No. 4,135,115 issued to Abernethy, et al. sets forth a WATTAGE REDUCING DEVICE FOR FLUORESCENT FIXTURES comprising the combination of a step-up transformer, a resistor and two capacitors, all of which are mounted externally of the ballast. The device is wired in series with the ballast and one of the lamps to allow normal ballast voltages to be delivered to the lamp circuit.
U.S. Pat. No. 4,859,914 issued to Summa sets forth a HIGH FREQUENCY ENERGY SAVING BALLAST which provides energizing signals characterized by frequencies in the range from about sixty hertz to thirty megahertz. An oscillator and transformer provide the energizing signals which are transformer-coupled to the lamp circuits.
U.S. Pat. No. 4,870,340 issued to Kral sets forth a METHOD AND APPARATUS FOR REDUCING ENERGY CONSUMPTION which includes switching apparatus for switching the load voltage off at arbitrary positions in the sine wave of the AC power applied while simultaneously providing a commutating path for any inductive current.
U.S. Pat. No. 4,965,492 issued to Boldwyn sets forth a LIGHTING CONTROL SYSTEM AND MODULE which includes a microprocessor control utilized to operate the lighting system at reduced power level while maximizing efficiency. The microprocessor and control circuitry continuously monitors the power applied and maintains the desired power level to maintain the preestablished light level selected.
While the foregoing described prior art systems have in various ways achieved energy saving and in many instances improved lighting characteristics, they are often complex and expensive to install and maintain. Thus, there remains a continuing need in the art for evermore improved and reliable lighting control systems which provide energy savings to the consumer.
In recognition of this need, the subject assignee has previously developed an improved lighting controller as disclosed in U.S. Pat. No. 5,442,261. Although such system has proven generally effective, there exists a need to prevent power interruption to the load and high transient current circulating through the components of the system during the switching from one voltage level to the other, without having recourse to using expensive components. A power interruption to the load when the system switches from full voltage to reduced voltage would cause the plasma in the fluorescent or high-intensity discharge lamps to quench and require a start-up cycle at full voltage to reheat.
The present invention addresses the above problem by providing a system which utilizes inexpensive components to perform the voltage switching function without power interruption to the load and without high current circulating through the components during the voltage switching.